Linear-low-density polyethylene(LLDPE) prepared by ionic polymerization of ethylene and .alpha.-olefin exhibits mechanical properties, such as impact strength, tensile strength and resistance for environmental stress-cracking that are superior to those of low-density polyethylene(LDPE) prepared by radical polymerization, and has therefore been used widely as materials for films, blow moldings and injection moldings.
LLDPE has, however, certain undesirable properties such as low melt tension and low transparency, and, because of these properties, its use is restricted. For example, when the polymers of a low melt tension are subjected to the conventional inflation film molding processes for manufacturing of films, so unstable bubbles are formed that films can hardly be manufactured. Moreover, when the polymers of a low melt tension are used in the blow molding processes, the extruded molten parison will deform by its own gravity.
The most common approach to improve the melt tension and transparency of LLDPE has been to blend LLDPW with LDPE. However, this approach has a defect in that the addition of LDPE to LLDPE sacrifices the mechanical strength of LLDPE, and that more man power and time are required for the blending operation.
Recently, there have been demands for improved ethylene-.alpha.-olefin copolymers having a high melt tension and transparency, and retaining good mechanical properties such as high impact strength, high tensile strength and high modulus.
The present inventors have extensively investigated seeking improved ethylene-.alpha.-olefin copolymers with the above mentioned properties, and, as a result of the investigation, have found that an ethylene-.alpha.-olefin copolymer, of which ratio of carbon-carbon double bonds in the form of trans-vinylene (trans-vinylene type carbon-carbon double bonds) to the total carbon-carbon double bonds (hereinafter referred to as TVR) is not less than 35%, possesses a high melt tension and a high transparency as well as high methanical properties.